Both the LCD monitor and LCD TV apparatus use the cold cathode fluorescent lamp as a backlight because this lamp has the best illumination efficiency. Therefore, large-size LCD panels usually contain multiple cold cathode fluorescent lamps. There are various methods of implementing a DC/AC inverter to drive the multiple cold cathode fluorescent lamps. Topologies such as half-bridge, full-bridge, and push-pull circuits are examples.
FIG. 1 shows a schematic drawing of a conventional center-tap primary windings push-pull topology used to drive cold cathode fluorescent lamps in accordance with the prior art. DC power 110 provides DC power to the push-pull circuit. DC power 110 is connected to the primary windings 810-814 of the transformers 310-314 which are connected in parallel. Each secondary winding 710-714 of the transformers 310-314 is coupled to a cold cathode fluorescent lamp circuit 410-414. There is a center-tap on each primary winding 810-814. Two power switches 212 and 214 are coupled to each primary winding 810-814. However, conventional push-pull circuit is limited due to undesired voltage spikes which are caused by the leakage-inductance energy of the transformer at the power switches when they are turned off. In addition, for a high-voltage application, the transformer primary winding needs more turns than those in battery-input applications, which will increase the size and the cost of the transformer. And the transformer winding ratio of FIG. 1 is:Ratio≈VOUTrms/VIN;  (1)Where VOUTrms is the maximum output voltage and VIN is the minimum input voltage of the transformer. The voltage spike across the switch is usually suppressed by a snubbed circuit to absorb the leakage energy. This passive implementation reduces the power conversion efficiency and increases the system cost with additional parts.
FIG. 2 shows another schematic drawing of a conventional half-bridge topology used to drive cold cathode fluorescent lamps in accordance with the prior art. DC power 110 provides DC power to the half-bridge circuit. Two switches 212 and 214 are coupled to each primary winding 810-814 of the transformers 310-314. Each secondary winding 710-714 of the transformers 310-314 is coupled to a cold cathode fluorescent lamp circuit 410-414. However, for the bridge-type (either half-bridge or full-bridge) circuitry, a level shifter circuit and a high-side driver is needed for the power transistor connected to the input voltage source. For high-voltage applications, this will increase the circuit cost significantly and suffers reliability of switching high-voltage signals.